this seams to be a problem when using Power Mailer under HTTPS. to fix the issue, edit the admin_pm/common/sendPreview.php file, and change line 179:
$httpURL = preg_replace("/^http:\/\//i", "", $WAGLOBAL_Root_URL.'admin_pm');
to:
$httpURL = preg_replace("/^https:\/\//i", "", $WAGLOBAL_Root_URL.'admin_pm');